Electorode wire for electrical discharge machining apparatus

ABSTRACT

It is an object of the invention to provide an electrode wire for an electrical discharge machining apparatus, which is low-priced in cost of production, has sufficient conductivity and strength at high temperature and is suited for improving the speed of electrical discharge machining. Cu—Zn alloy covering layer is formed around a core metallic wire formed of Cu-0.02 to 0.2 Zr alloy or Cu-0.15 to 0.25 Sn-0.15 to 0.25 In alloy.

FIELD OF THE INVENTION

[0001] This invention relates to an electrode wire for an electricaldischarge machining apparatus, and especially to an electrode wire foran electrical discharge machining apparatus having a covering layer.

BACKGROUND OF THE INVENTION

[0002] As a conventional electrode wire for an electrical dischargemachining apparatus, a Cu—Zn alloy wire (a brass wire) containing Zn of32 to 36 weight percent is used.

[0003] Besides this, a composite wire composed of a core metallic wireformed of a steel wire and a covering layer formed of Cu-35 Zn alloy isknown as an electrode wire for an electrical discharge machiningapparatus with height strength. Moreover, an electrode wire for thesame, which is composed of a core metallic wire formed of Cu alloy, suchas Cu-0.15Sn or Cu-0.15Ag alloy, and a covering layer formed of Cu-35Znalloy is known (Japanese Patent Kokai No.6-47130).

[0004] As a method for increasing electrical discharge machining speedand obtaining an electrode wire for the same with high efficiency, amethod, in which concentration of Zn of Cu—Zn alloy is increased orheat-resisting property of the electrode wire is improved by adding Alto Cu—Zn alloy, is known (Furukawa Electric Review, No.75, March, 1985).

[0005] Recently, further elevation of electrical discharge machiningspeed is demanded from a view point of improvement of productivity. Inorder to meet the aforementioned demand, an electrode wire with coveringlayer for an electrical discharge machining apparatus, which is composedof a core metallic wire formed of Cu-2.0Sn, Cu-0.3Sn, Cu-13Zn, Cu-0.6Agor Cu-4.0Zn-0.3Sn and a covering layer formed of a Cu—Zn alloycontaining Zn of high weight percent, is proposed (Japanese Patent KokaiNo.5-339664).

[0006] However, in the aforementioned electrode wire for the electricaldischarge machining apparatus, since the Cu—Zn alloy layer contains Znof 38 to 49 weight percent, the Cu—Zn alloy layer is formed of a mixingcomposition of α and β phases, or a single phase composition of only βphase. Since cold working of the Cu—Zn alloy layer becomes difficult asa composition of β phase becomes dominant, the aforementioned electrodewire for the electrical discharge machining apparatus can be producedonly by hot working (hot extrusion), hence production cost thereofbecomes high.

[0007] Moreover, in the aforementioned electrode wire for the electricaldischarge machining apparatus, since Cu alloy, such as Cu-2.0Sn,Cu-0.3Sn, Cu-13Zn, Cu-0.6Ag or Cu-4.0Zn-0.3Sn, is adopted as material ofthe core metallic wire, following disadvantages are inevitable. Thisproduct is detective in workability in the process of drawing in casethat the core metallic wire is formed of Cu-2.0Sn. Heat-resistingproperty (strength at high temperature) of the product is low, and atthe time of practical use, instability of discharge occurs because ofthe breaking of a wire or the elongation of the wire before the breakingin case that the core metallic wire is formed of Cu-13Zn. In the productwith low electrical conductivity (in case that the core metallic wire isformed of Cu-4.0Zn-0.35Sn) or low heat-resisting property, improvementof electrical discharge machining speed is not satisfactory. In case ofalloy containing Ag, material cost becomes high. Referring to the coremetallic wire, since heat-resisting property of a Cu alloy disclosed inJapanese Patent Kokai No.6-47130 is insufficient, electrical dischargemachining speed cannot be improved (Cu-0.15Sn), and material cost of Cualloy containing Ag is high in general.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of the invention to solve theaforementioned problems and provide an electrode wire for an electricaldischarge machining apparatus composed of a core metallic wire formed ofCu alloy and a covering layer formed of Cu—Zn alloy, which is low pricedin material cost, has sufficiently high electrical conductivity andheat-resisting property and is suited for improving electrical dischargemachining speed.

[0009] It is a further object of the invention to provide an electrodewire for an electrical discharge machining apparatus, which is composedof a core metallic wire formed of Cu alloy and a Cu—Zn alloy coveringlayer formed of a single phase composition of only α phase.

[0010] It is a still further object of the invention to provide anelectrode wire for an electrical discharge machining apparatus, which iscomposed of a core metallic wire formed of Cu alloy and a Cu—Zn alloycovering layer formed of a mixing phase composition of α and β phases.

[0011] According to the first feature of the invention, an electrodewire for an electrical discharge machining apparatus comprises:

[0012] a core metallic wire formed of Cu-0.02 to 0.2Zr alloy or Cu-0.15to 0.25Sn-0.15 to 0.25In in alloy, and

[0013] a covering layer formed of Cu—Zn alloy.

[0014] According to the second feature of the invention, an electrodewire for an electrical discharge machining apparatus comprises:

[0015] a core metallic wire formed of Cu-0.02 to 0.2 Zr alloy or Cu-0.15to 0.25Sn-0.15 to 0.25 In alloy, and

[0016] a Cu—Zn alloy covering layer formed of a single phase compositionof only α phase.

[0017] According to the third feature of the invention, an electrodewire for an electrical discharge machining apparatus comprises:

[0018] a core metallic wire formed of Cu-0.02 to 0.2Zr alloy or Cu-0.15to 0.25Sn-0.15 to 0.25 In alloy, and

[0019] a Cu—Zn alloy covering layer formed of a mixing composition of αand β phases.

[0020] The invention pays the attention to material of a core metallicwire of an electrode wire for an electrical discharge machiningapparatus having a covering layer formed of Cu—Zn alloy.

[0021] The reason for limiting material of the core metallic wire toCu-alloy is that tensile strength and electrical conductivity at hightemperature is satisfactory. A steel wire is omitted, because it isdefective in straightness, when it comes loose. Moreover, it isdifficult to apply the steel wire to a processing machine. A Cu wire isomitted because tensile strength thereof at high temperature isinsufficient.

[0022] The reason for selecting the aforementioned numerical values oncomposition of the core metallic wire will be explained.

[0023] In Cu-0.02 to 0.2Zr alloy, when concentration of Zr is less than0.02 weight percent, heat-resisting property of alloy is insufficientand instability of discharge arises, and when concentration of Zr ismore than 0.2 weight percent, it exceeds the limit of solid solution ofCu—Zn alloy and precipitation of Cu₃Zr starts, and the breaking of awire is apt to occur, so that concentration of Zr is limited within arange of 0.02 to 0.2 weight percent. Since Cu-0.05 to 0.16Zr alloy, inwhich concentration of Zr is 0.05 to 0.16 weight percent, is widely usedfor various purposes as Cu-0.16Zr alloy, this alloy is the mosteconomical in Cu—Zr alloy.

[0024] Next, concentrations of Sn and In in Cu-0.15 to 0.25Sn-0.15 to0.25 In alloy will be discussed. Sn and In are added to alloy in orderto increase the strength of alloy, but the effect of Sn on a decrease ofthe electrical conductivity of alloy is more noticeable than that of In.Since the electrical conductivity of the wire should be kept to be highfrom a view point of stability of discharge characteristic, it isdesirable that concentration of In is higher that of Sn. However, sinceIn is high-priced, concentration of In is kept to be less than 0.25%.Accordingly, there is necessity to increase the amount of addition ofSn, but the conductivity of alloy noticeably decreases in case thatconcentration of Sn is more than 0.25 weight percent. The aforementionedcomposition is selected on the basis of trade-off between improvement ofdischarge characteristic and economical consideration.

[0025] Moreover, concentration of Zn of Cu—Zn alloy will be discussed.In case that concentration of Zn is 32 to 38 weight percent, Cu—Zn alloycan be formed of a single phase composition of α phase, and in theregion of α phase, although tensile strength and hardness increases asconcentration of Zn increases, hardness is not so high and Cu—Zn alloycan be processed by cold working. Accordingly, manufacturing processincluding drawing is easily carried out. Concentration of Zn of 32 to 36weight percent corresponds to that of Cu-35Zn alloy (65/35 brass), whichis widely used for various purposes. Cu-35Zn alloy is formed of a singlephase composition of α phase, suited for cold working, easily obtainedon the market and favorable from a viewpoint of economy.

[0026] Furthermore, since the thickness of the covering layer formed ofCu—Zn alloy is consumed by about 30 μm in an electrical dischargemachining process of high efficiency, the thickness of the coveringlayer formed of Cu—Zn alloy is selected to be more than 30 μm in orderto avoid a situation that the breaking of a wire occurs, and less than40 μm, because the electrical conductivity of the wire becomesinsufficient for fulfilling the function of an electrode wire for anelectrical discharge machining apparatus in case that the thickness ofthe covering layer is more than 40 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention will be explained in more detail in conjunctionwith appended drawings, wherein:

[0028]FIG. 1 is a cross-sectional view of an electrode wire for anelectrical discharge machining apparatus having a covering layer,

[0029]FIG. 2A is a photograph for showing a composition of across-section of an electrode wire for an electrical discharge machiningapparatus according to the fourth preferred embodiment, and

[0030]FIG. 2B shows distribution of concentration of Zn in a Cu—Znalloy-covering layer of the fourth preferred embodiment.

DISCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Thereafter, the preferred embodiments of the invention will beexplained.

[0032]FIG. 1 shows a cross-sectional view of an electrode wire for anelectrical discharge machining apparatus according to the invention.

[0033] As shown in FIG. 1, the electrode wire 3 for the electricaldischarge machining apparatus according to the invention is composed ofa core metallic wire 1 formed of Cu-0.02 to 0.2 Zr alloy (or Cu-0.15 to0.25 Sn-0.15 to 0.25 In alloy) and a covering layer 2 formed of Cu—Znalloy, which is formed of a single phase composition of α phase or amixing composition of α and β phases.

[0034] According to an electrode wire for an electrical dischargemachining apparatus, which is composed a core metallic wire formed ofCu-0.02 to 0.2 Zr alloy or Cu-0.15 to 0.25 Sn-0.15 to 0.25 In alloy anda covering layer formed of Cu—Zn alloy being formed of a single phasecomposition of α phase, according to the invention, the electrode wirefor the electrical discharge machining apparatus with high efficiency,which can be easily processed by cold working, can be obtained. That isto say, since the Cu—Zn alloy covering layer according to the inventionis formed of a single phase composition of α phase, the mechanicalproperty of the Cu—Zn alloy covering layer according to the invention isdifferent from that of a conventional Cu—Zn alloy covering layer, whichis formed of a mixing composition of α and β phases or a single phasecomposition of only β phase. Tensile strength of the former is smallerthan that of the latter, and reduction of area of the former is largerthan that of the latter, so that deformability of the former is higherthan that of the latter. In case that Cu—Zn alloy covering layer isformed of a single phase composition of α phase, the improvement ofelectrical discharge machining speed is small as compared with case thatCu—Zn alloy layer is formed of a mixing composition of α and β phases oronly β phase, because concentration of Zn is reduced. However, in caseof the electrode wire according to the invention, depreciation of theimprovement of electrical discharge machining speed is made to be assmall as possible by adopting the electrode wire having a core metallicwire formed Cu-0.02 to 0.2 Zr or Cu-0.15 to 0.25 Sn 0.15 to 0.25 Inalloy. Although the Cu—Zn alloy covering layer is formed of a singlephase composition of α phase, electrical discharge machining speed ofthe electrode wire for the electrical discharge machining apparatusaccording to the invention is far higher than that of the conventionalelectrode wire for the electrical discharge machining apparatus formedof solid Cu-35Zn alloy.

[0035] Since the electrode wire can be manufactured by cold working, theelectrode wire thus obtained is low-priced and cost of production of theelectrode wire for the electrical discharge machining apparatus can bereduced in turn.

[0036] When Cu-35Zn alloy, which is easily obtainable on the market, thelowest in price and widely used for various purposes, is selected amongCu—Zn alloy products, cost of production of the electrode wire for theelectrical discharge machining apparatus can be further reduced.

[0037] On the other hand, in case that a Cu—Zn alloy covering layer iscomposed of a mixing composition of α and β phases, although theimprovement of electrical discharge machining speed is high, forconcentration of Zn is high, it becomes difficult to process the Cu—Znalloy covering layer by cold working, as percentage of β phasecomposition increases. In such a case, if concentration of Zn near thesurface of the Cu—Zn alloy covering layer is made to be low by heattreatment, the Cu—Zn alloy covering layer can be easily processed bycold working, though the content of Zn is high as a whole. Since it isextremely difficult to process a Cu—Zn alloy covering layer formed of asingle phase composition of only β phase by cold working, this alloylayer is in the outside of the scope in the invention.

[0038] (The First Preferred Embodiment)

[0039] A composite wire is formed by inserting a core metallic wire,which is formed of Cu-0.16 Zr alloy and has an external diameter of 7.1mm, into a pipe, which is formed of Cu-35 Zn alloy and has an externaldiameter of 10 mm and a pipe thickness of 1.2 mm. The Cu-35 Zn alloypipe is manufactured by ordinary extrusion process.

[0040] The composite wire is formed into that with a diameter of 0.9 mmby drawing, to which heat treatment is applied for annealing.

[0041] Finally, the composite wire with a diameter 0.9 mm is drawn to beformed into that with a diameter of 0.25 mm, and an electrode wire foran electrical discharge machining apparatus, which comprises a Cu—Znalloy covering layer with a thickness of 31 μm containing Zn of 35weight percent, can be obtained.

[0042] Moreover, a composite wire is formed by inserting a core metallicwire, which is formed of Cu-0.16 Zr and has an external diameter of 7.1mm, into a pipe, which is formed of Cu-40 Zn alloy and has an externaldiameter of 10 mm and a pipe thickness of 1.2 mm. The Cu-40 Zn alloypipe is manufactured by ordinary extrusion process.

[0043] The composite wire is formed into that with a diameter of 7.9 mmby drawing, to which heat treatment is applied for annealing. Next, thecomposite wire is formed into that with a diameter of 1.2 mm by drawing,to which heat treatment is again applied for annealing.

[0044] Finally, the composite wire with a diameter of 1.2 mm is drawn tobe formed into that with a diameter of 0.25 mm, and an electrode wirefor an electrical discharge machining apparatus, which comprises a Cu—Znalloy covering layer with a thickness of 31 μm containing Zn of 40weight percent.

[0045] (The Second Preferred Embodiment)

[0046] The manufacturing process of an electrode wire for an electricaldischarge machining apparatus according to the second preferredembodiment is the same as that of the first preferred embodiment exceptthat a core metallic wire is formed of Cu-0.19Sn-0.2In alloy and anexternal diameter thereof is 7.1 mm, and electrode wires for theelectrical discharge machining apparatus of two kinds, the Cu—Zn alloylayers of which respectively contain Zn of 35 and 40 weight percent andare commonly 31 μm thick, are manufactured.

EXAMPLES FOR COMPARISON 1

[0047] Two kinds of electrode wires for an electrical dischargemachining apparatus, an manufacturing process of which is the same asthat of the first preferred embodiment except that a core metallic wireis formed of Cu-0.2Sn alloy and an external diameter thereof is 7.1 mm,are manufactured as examples for comparison. The Cu—Zn alloy layers ofthe aforementioned electrode wires of two kinds respectively contain Znof 35 and 40 weight percent and are commonly 31 μm thick. It is foundthat the core metallic wire formed of Cu-2.0Sn alloy is not suited fordrawing process, and the manufacturing process of the electrode wire forthe discharge machining apparatus comprising this alloy layer is notsmooth.

EXAMPLE FOR COMPARISON 2

[0048] Two kinds of electrode wires for an electrical dischargemachining apparatus, a manufacturing process of which is the same asthat of the first preferred embodiment except that a core metallic wireis formed Cu-0.3Sn alloy and an external diameter thereof is 7.1 mm, aremanufactured as examples for comparison. The Cu—Zn alloy layers of theaforementioned electrode wires of two kinds respectively contain Zn of35 and 40 weight percent and are commonly 31 μm thick.

EXAMPLES FOR COMPARISON 3

[0049] Two kinds of electrode wires for an electrical dischargemachining apparatus, a manufacturing process of which is the same asthat of the first preferred embodiment except that a core metallic wireis formed of Cu-0.15Sn alloy and an external diameter thereof is 7.1 mm,are manufactured as examples for comparison. The Cu—Zn alloy layers ofthe aforementioned electrode wires of two kinds respectively contain Znof 35 and 40 weight percent and are commonly 31 μm thick.

EXAMPLES FOR COMPARISON 4

[0050] Two kinds of electrode wires for an electrical dischargemachining apparatus, a manufacturing process of which is the same asthat of the first preferred embodiment except that a core metallic wireis formed of Cu-0.13Sn alloy and an external diameter thereof is 7.1 mm,are manufactured as examples for comparison. The Cu—Zn alloy layers ofthe aforementioned electrode wires of two kinds respectively contain Znof 35 and 40 weight percent and are commonly 31 μm thick.

EXAMPLES FOR COMPARISON 5

[0051] Two kinds of electrode wires for an electrical dischargemachining apparatus, a manufacturing process of which is the same asthat of the first preferred embodiment except that a core metallic wireis formed of Cu-4.0Zn-0.3Sn alloy and an external diameter thereof is7.1 mm, are manufactured as examples for comparison. The Cu—Zn alloylayers of the aforementioned electrode wires of two kinds respectivelycontain Zn of 35 and 40 weight percent and are commonly 31 μm thick.

EXAMPLES FOR COMPARISON 6

[0052] Two kinds of electrode wires for an electrical dischargemachining apparatus, a manufacturing process of which is the same asthat of the first preferred embodiment except that a core metallic wireis formed of Cu-0.6Ag alloy and an external diameter thereof is 7.1 mm,are manufactured as examples for comparison. The Cu—Zn alloy layers ofthe aforementioned electrode wires of two kinds respectively contain Znof 35 and 40 weight percent and are commonly 31 μm thick.

EXAMPLES FOR COMPARISON 7

[0053] Two kinds of electrode wires for an electrical dischargemachining apparatus, a manufacturing process of which is the same asthat of the first preferred embodiment except that a core metallic wireis formed of Cu-0.15Ag alloy and an external diameter thereof is 7.1 mm,are manufactured as examples for comparison. The Cu—Zn alloy layers ofthe aforementioned electrode wires of two kinds respectively contain Znof 35 and 40 weight percent and are commonly 31 μm thick.

A CONVENTIONAL EXAMPLE 1

[0054] An electrode wire for an electrical discharge machining apparatuswith an external diameter of 0.25 mm formed of only Cu-35Zn alloy ismanufactured.

A CONVENTIONAL EXAMPLE 2

[0055] An electrode wire for an electrical discharge machining apparatuswith an external diameter of 0.25 mm formed of only Cu-40Zn alloy ismanufactured.

[0056] Table 1 shows the data of the core metallic wires used in thefirst to second preferred embodiments, the examples for comparison 1 to7 and the examples of the conventional electrode wires 1 to 2. The unitsof chemical compositions shown in table 1 are given by weight percent.

[0057] (Table 1)

[0058] Next, strengths at high temperature (MPa), conductivities (%IACS) and workabilities in drawing of the core metallic wires used inthe first to second preferred embodiments, the examples for comparison 1to 7 and the examples of the conventional electrode wires 1 to 2 areestimated. The results of the comparison are shown in table 1.

[0059] Strengths of the wires at high temperature are estimated asfollow. Each core metallic wire with a diameter of 7.1 mm is deformedinto that with a diameter of a 0.2 mm by drawings, and thereafter thetemperature of the core metallic wire is kept to be 300° C. for tenminutes. The temperature of 300° C. corresponds to that of the coremetallic wire of the electrode wire for the electrical dischargemachining apparatus, when it is actually used for its essential purpose.Thereafter, a tensile strength of the core metallic wire is estimated.Electrical conductivity of the core metallic wire is measured afterdrawing process. The workability in the drawing process is estimated byactually drawing the core metallic wire by dices. The drawings arerepeated by inserting heat treatments therebetween at need, and theworkability is estimated on the basis of occurrences of the breaking ofthe wire, the degree of reduction of the cross-sectional area of thewire per one pass and the limit of reduction rate. A double circle, asingle circle and a triangle respectively mean that the workability isexcellent, acceptable and poor.

[0060] Next, electrical discharge machining speeds of the electrodewires for the electrical discharge machining apparatus according to thefirst to second preferred embodiments, the examples for comparison 1 to7 and the examples of the conventional electrode wires 1 to 2 areestimated. The results of the estimation are shown in table 1.

[0061] Electrical discharge machining speed is measured on a work (asample to be machined, JIS SKD-11) with a dimension of 60 mm by using anelectrical discharge machining tester (FX10, manufactured by MitsubishiElectric Co.). Electrical discharge machining speed is normalized on thebasis of that of the conventional electrode wire 1 (2.184 mm/min.).

[0062] As shown in table 1, electrical discharge machining speeds of theelectrode wires for the electrical discharge machining apparatusaccording to the first to second referred embodiments and the examplesfor comparison 1 to 7 are noticeably improved as compared with those ofthe conventional electrode wires 1 to 2. It can be presumed that theaforementioned results are attributable to the fact that the electrodewire for the electrical discharge machining apparatus with the coveringlayer is adopted. That is to say, it is useful for the advance oftechnology to adopt the electrode wire for the electrical dischargemachining apparatus having a covering layer in stead of that formed ofsolid alloy, and thereby the speed of electrical discharge machining isincreased.

[0063] On the electrode wires for the electrical discharge machiningapparatus according to the first to second preferred embodiments and theexamples for comparison 1 to 7, which respectively have the differentqualities of the material, the properties of the core metallic wires andthe improvements of electrical discharge machining speeds will becollectively discussed.

[0064] In both the cases of the electrode wires for the electricaldischarge machining apparatus according the first and second preferredembodiments, both strength at high temperature and electricalconductivity are satisfactory, and excellent results can be confirmed.

[0065] On the other hand, in case of an electrode wire for an electricaldischarge machining apparatus according an example for comparison 1,workability in drawing is defective, and it is difficult to manufacturethe electrode wire for the electrical discharge machining apparatus.

[0066] In case of an electrode wire for an electrical dischargemachining apparatus according to an example for comparison 2,conductivity is insufficient and the improvement of the speed ofelectrical discharge machining is not satisfactory.

[0067] In case of an electrode wire for an electrical dischargemachining apparatus according to an example for comparison 3, strengthat high temperature is insufficient, and the improvement of theelectrical discharge machining speed is not satisfactory.

[0068] In case of an electrode wire for an electrical dischargemachining apparatus according to an example for comparison 4, strengthat high temperature is extremely low, and the breaking of a wire mayoccur at the time of electrical discharge machining.

[0069] In case of an electrode wire for an electrical dischargemachining apparatus according to an example for comparison 5, electricalconductivity is low, and the improvement of electrical dischargemachining speed is insufficient.

[0070] In case of an electrode wire for an electrical dischargemachining apparatus according to an example for comparison 6 and 7,material of the core metallic wire contains Ag, and cost of material ishigh in general.

[0071] (The Third Preferred Embodiment)

[0072] A composite wire is formed by inserting a core metallic wire,which is formed of Cu-0.16Zr alloy and has an external diameter of 7.1mm, into a pipe, which is formed of Cu-40Zn alloy and has an externaldiameter of 10 mm and a pipe thickness of 1.2 mm.

[0073] This composite wire is formed into that with a diameter of 7.9 mmby drawing, to which heat treatment is applied at 450° C. for 1 hr.Next, the composite wire with a diameter of 7.9 mm is formed into thatwith a diameter of 1.2 mm by drawing, to which heat treatment is appliedat 450° C. for 1 hr.

[0074] Finally, the composite wire with a diameter of 1.2 mm is formedinto that with a diameter of 0.25 mm by drawing, to which heat treatmentis applied. An electrode wire for an electrical discharge machiningapparatus having a structure mentioned as follows can be obtained bycontrolling the extent of heat treatment. Concentration of Zn in theCu—Zn alloy covering layer varies in the radial direction within a rangeof 35 to 45 weight percent. Concentration of Zn in a shallow layer,which is about 5 μm deep below the surface of the covering layer, islower than that in a deep layer, and the total thickness of the Cu—Znalloy covering layer is about 31 μm.

[0075] (The Fourth Preferred Embodiment)

[0076] The manufacturing process of the fourth preferred embodiment isthe same as that of the third preferred embodiment except that a coremetallic wire is formed of Cu-0.19Sn-0.2In alloy and an externaldiameter thereof is 7.1 mm. An electrode wire for an electricaldischarge machining apparatus having a structure mentioned as follow canbe obtained by controlling the extent of heat treatment. Concentrationof Zn in the Cu—Zn alloy covering layer varies in the radial directionwithin a range of 35 to 45 weight percent. Concentration of Zn in ashallow layer, which is about 5 μm deep below the surface of thecovering layer, is lower that in a deep layer, and the total thicknessof the Cu—Zn alloy covering layer is about 31 μm.

[0077] As shown in FIG. 2B, in each of the electrode wires for theelectrical discharge machining apparatus according to the third andfourth preferred embodiments, since concentration of Zn in a shallowlayer, which is about 5 μm deep below the surface of the Cu—Zn alloycovering layer, is about 35 weight percent, workability in cold workingis improved and drawing process at room temperature can be easilycarried out, although there is a layer containing Zn of highconcentration in an inner portion of the Zn—Cu alloy covering layer.

[0078]FIGS. 2A and 2B show the results obtained in case of the electrodewire for the electrical discharge machining apparatus according to thefourth preferred embodiment. It is a matter of course that similarresults can be obtained in case of the same according to the thirdpreferred embodiment.

[0079] As mentioned in the above, in case of the electrode wire for theelectrical discharge machining apparatus according to the invention,since the Cu—Zn alloy covering layer is formed around the core metallicwire, which is formed of Cu-0.02 to 0.2 Zr alloy or Cu-0.15 to 0.25Sn-0.15 to 0.25 In alloy, cost of production is low, satisfactoryelectrical conductivity and strength at high temperature can be obtainedand speed and efficiency of electrical discharge machining ca beimproved as compared with the conventional electrode wire for theelectrical discharge machining apparatus comprising the known coremetallic wire.

[0080] Although the invention has been described with respect tospecific embodiment for complete and clear disclosure, the appendedclaims are not to be thus limited but are to be construed as embodyingall modification and alternative constructions that may be occurred toone skilled in the art which fairly fall within the basic teaching hereis set forth.

What is claimed is:
 1. An electrode wire for an electrical dischargemachining apparatus comprising: a core metallic wire formed of Cu-0.02to 0.2 WT % Zr alloy or Cu-0.15 to 0.25 WT % Sn-0.15 to 0.25 WT % Inalloy, and a covering layer formed of Cu—Zn alloy.
 2. An electrode wirefor an electrical discharge machining apparatus, comprising: a coremetallic wire formed of Cu-0.02 to 0.2 WT % Zr alloy or Cu-0.15 to 0.25WT % Sn-0.15 to 0.25 WT % In alloy, and a Cu—Zn alloy covering layerformed of a single phase composition of only α phase.
 3. An electrodewire for an electrical discharge machining apparatus according to claim2 , wherein: said Cu—Zn alloy covering layer contains Zn of 32 to 38weight percent.
 4. An electrode wire for an electrical dischargemachining apparatus according to claim 1 , wherein: a thickness of saidCu—Zn alloy covering layer is 30 to 40 μm.
 5. An electrode wire for anelectrical discharge machining apparatus, comprising: a core metallicwire formed of Cu-0.02 to 0.2 WT % Zr alloy or Cu-0.15 to 0.25 WT %Sn-0.15 to 0.25 WT % In alloy, and a Cu—Zn alloy covering layer formedof a mixing composition of α and β phases.
 6. An electrode wire for anelectrical discharge machining apparatus according to claim 5 , wherein:concentration of Zn of said Cu—Zn alloy covering layer in a shallowlayer below a surface of said Cu—Zn alloy covering layer is lower thanthat in an inner portion of said Cu—Zn alloy covering layer.